RAMAN SCATTERING INTRODUCTION The Raman effect is the ...

RAMAN SCATTERING
INTRODUCTION
The Raman effect is the appearance of weak lines in the spectrum of light scattered by a
substance which has been illuminated by a monochromatic light (with angular frequency ω).
The lines occur close to, and on each side of, the incident light frequency, and hence are optical
sidebands. The sidebands arise from the nonlinear interaction of the light with atomic or
molecular quantum states in the scattering material. In a classical picture, the light induces
a dynamic (time dependent) response in the polarizability of the substance, and then the
product of the polarizability with the original light field results in the optical sidebands. In a
quantum mechanical picture, the nonlinearity is equivalent to second order time-dependent
perturbation theory. In this case, one encounters a product involving a quantum state a
with time dependence exp (−iω a t), the complex conjugate of a quantum state b with time
dependence exp (iω b t) , and the electromagnetic field with time dependence cos(ωt). Using
simple trig identities, one obtains a resultant time dependence cos [(w − (ω b − ω a ))t] and
cos [(ω + (ω b − ω a ))t] . By analogy with the terminology used in fluorescence, the lines corresponding
to a lower frequency are called Stokes lines and those corresponding to a higher
frequency are called Anti-Stokes lines. By measuring the frequency shifts and ω b − ω a , the
structure of the system can be determined. [1–10]
Recalling that second order perturbation
theory involves a sum over virtual
states, a pictorial mnemonic for Raman
scattering may be viewed as in Fig. 1.
Figure 1. Illustration of the quantum
state transitions for the Stokes (left)
and Anti-Stokes (right) processes.
Carbon tetrachloride provides a good example of a Raman sample; the low-lying levels are
different vibrational states of the molecule and the virtual state lies near an excited electronic
state of the molecule. By examining the Raman spectrum, the frequency of the vibrational
modes of the molecule can be deduced. Additional information can be obtained from the
strength of the various lines and the polarization dependence of the spectra, which may be
found from the details of the time dependent perturbation theory.
Since the Raman effect is second order (nonlinear), the effect is weak, and a strong source of
incident light is required. The Raman experiment uses a powerful argon (Ar) ion laser as the
incident source, and the weak Raman sidebands are detected with a double monochromator
scanning spectrometer and a sensitive photomultiplier tube (PMT) which is cooled to reduce
its intrinsic thermal noise.
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EQUIPMENT
Oriel 6035 Hg(Ar) lamp with reference “Typical Spectra of Oriel Instruments”
Coherent 31-2017 He-Ne laser (JDS Uniphase) and power supply
IonLaser Technologies 5500A Ar ion laser
Spex 1700-111 Spectrometer for measuring light spectra, with scanning power supplies
Photomultiplier Tube (photomultiplier tutorial)
Products for Research 375 photomultiplier socket and TE104 housing attached to
spectrometer and amplifier, with PC104CE Peltier cooler
Physics Electronics Shop HV Supply, Amplifier and Discriminator
Agilent 53131A Universal Counter (operating guide, programming guide and
service guide) (GPIB address 3)
Computer with National Instruments LabVIEW and a data acquisition computer program
(see C:\General Files\Raman files\raman datacq jdm.pdf)
Lenses and samples (including carbon tetrachloride)
Reference: Raman Handbook
CAUTION: The Ar ion Laser can burn eyes, fingers, clothing, etc. Laser safety goggles
must be worn at all times. Do not put shiny objects into the beam. Use care when aligning
or realigning mirrors. Do all alignments at minimum power (see laser manual for details of
power adjustment). Use bent pipes as optical dumps to stop stray laser light.
In this experiment, the Raman spectra will be measured for several liquids, starting with
carbon tetrachloride (CCl 4 ). Other liquids include chloroform (CHCl 3 ) and dichloromethane
(CH 2 Cl 2 ).
CAUTION: The Raman samples are contained in sealed glass vials, because they are
hazardous and create noxious odors. Be careful when handling the samples. It may be
necessary to prepare fresh samples.
An illustration of the Raman scattering experiment is shown in Fig. 2. A moderate power
He-Ne laser is used to align the optical system. The beam from the powerful Ar ion laser is
deflected upward by a mirror so as to pass through the Raman sample (not shown in this
figure; the sample would be at the height of the He-Ne laser). Two lenses are used to focus
an image of the scattered light inside the illuminated sample onto a narrow opening (slit)
on the spectrometer. As discussed in the spectrometer manual, the spectrometer has three
adjustable slits: an entrance slit, an internal slit, and an exit slit. The photomultiplier unit,
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which is optically sealed to the exit slit, consists of the PMT, a housing, a socket base with
coaxial connections for the PMT, and a thermoelectric cooler with cables passing from the
housing.
Figure 2. A schematic of the Raman scattering experiment.
CAUTION: At times it may be necessary to open the cover of the spectrometer. Before
opening the spectrometer, make sure the shutter in front of the PMT is closed, otherwise the
PMT may be ruined. The control knob for the shutter is located below the spectrometer exit
slit. To close the shutter, turn the knob (it may be difficult to get it started) in the direction
for loosening a right-hand screw until it clicks into place and a black line is centered in view.
To open the shutter, turn in the direction for tightening a right-hand screw until it clicks
into place and a red line is centered in view. Use the shutter on the entrance slit (which is
more accessible) as a model to see how the shutter works.
CAUTION: Use only the side cover on the spectrometer (as shown in Fig. 2). Keep the
cover closed except when opening is absolutely necessary; dust is the biggest problem with
the spectrometer. Also, never touch the surface of any optical component; these surfaces
cannot be cleaned.
Other equipment not shown in Fig. 2 includes a) a computer for data acquisition; b) a device
made by the Physics Department Electronics Shop containing a high voltage (HV) power
supply connected the PMT cathode with a coaxial cable having special high-voltage BNC
connectors, a preamplifier and discriminator connected to the PMT anode (signal output)
with a BNC cable, and a BNC output for sending pulses to a frequency (pulse rate) counter;
c) a power supply connected to the the PMT thermoelectric cooler; and d) a scan control
unit for the spectrometer. Two small digital multimeters (DMM’s) are connected to the Ar
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laser and the PMT cooler power supply.
It should be noted that the scanning of wavelengths to produce the spectrum is accomplished
by rotating the spectrometer gratings with a motor controlled by the scan control unit. The
degree of rotation is indicated by a counter on the side of the spectrometer. Determining
a conversion from counter reading to wavelength (i.e. the calibration of the counter) is an
important part of the procedure.
PROCEDURE
1. The first step is to check the status of the PMT thermoelectric cooler. The cooler
is based on the thermoelectric effect, in which a DC electric potential difference generates
a temperature gradient; by anchoring the high end of the gradient near room
temperature with a flowing water heat exchanger, the low temperature end of the gradient
can be used to cool the PMT. The thermoelectric effect is reversible, so that a
temperature gradient can generate a potential difference. There are some difficulties
with this cooler, so that it cannot be turned on until two checks are performed. The
reason is that the cooler may have been used earlier, and the heat exchange water may
have been stopped when the cooler was turned off. This may result in two problems.
The first problem is that parts of the cooler which remain cold after shut-down may
cause the water (no longer flowing) to freeze and plug the heat exchanger. The second
problem is that the residual temperature gradient may establish an electric potential
across the leads connected to the cooler power supply, with the consequence that when
the power supply is turned on, the extra potential causes too much current to flow,
and a fuse is blown. The steps to check the status of the cooler are as follows:
1. Turn on the DMM connected to the PMT cooler power supply (which should be
off), and set it to measure DC voltage. The absolute value of the voltage should
be less than 0.1V; if not, there may be a residual temperature gradient on the
thermoelectric cooler, and you must wait until it reduces.
2. Follow the rubber tubing from the PMT housing to the source of the heat exchanger
water at a green-handled ball valve on a back wall near the floor. Turn
on the water by rotating the green handle until it is parallel to the outlet pipe. Do
not alter the round-handled throttling valve which is upstream from the greenhandled
ball valve. Water should exit the other rubber tube into a drain pipe,
and the lights on a safety flow switch mounted on the wall near the PMT housing
should change from red to green. If water does not flow, the PMT cooler may be
plugged with ice, and you will have to wait until it melts. If the water does flow,
turn it off by rotating the green handle until it is perpendicular to the outlet pipe.
Note that the water is filtered, and that the filter should be replaced if it appears
dirty.
If the checks indicate that the PMT cooler is operational, do not turn on the power
supply at this point. Also do not turn on the water at this point.
3. Check that the spectrometer exit slit shutter (at the PMT unit) is closed. With the
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PMT high voltage power supply turned off, make sure the HV SET knob is turned all
the way down (to zero). Turn on the PMT high voltage power supply and turn up the
HV SET knob until the voltage on the meter reaches 900 V (the actual output voltage
is negative); set the discriminator to 3.0. Using the PMT OUT and TRIGGER OUT
connections, check the pulses from the PMT with the oscilloscope. Note the nature
(shape, amplitude, width)of the pulses for your report. Also check the rear panel
PULSE OUT POSITIVE 4 V signal; connect this output to channel 1 of the frequency
counter.
4. Turn on the frequency counter which displays the pulse rate from the PMT (through
the HV supply, preamp and discriminator unit). Use channel 1, and use the “Other
Meas” botton to put the counter into the “TOTALIZE 1” mode. Set the gate time to
1 s, the auto-trigger to OFF and the trigger sensitivity to LO (see the Quick Reference
Guide in the manual). Based on the nature of the PULSE OUT POSITIVE 4 V signal
from the HV/preamp/discriminator unit, set the frequency counter input coupling,
trigger level and trigger slope. In your lab report, you should discuss why all of these
counter settings have their particular values.
5. Log on to the computer and start the Raman computer program by running the program
C:\General Files\Raman files\raman datacq jdm.vi. For now, the Spectrometer
Start Reading and Spectrometer End Reading are not necessary. Click the right arrow
at the upper-left of the display to start the program. The program’s “Photomultiplier
pulse rate” readout should match the display of the frequency counter.
With the PMT shutter closed, the count rate may have values in the range of a few
hundred to 1500 counts-per-second (cps). This is the room temperature PMT dark
current count rate. The data acquisition mode may be exited at the end of the run (or
at any earlier time) by clicking the Stop button. Leave it running for now.
6. Turn on the water to the PMT cooler, and turn on the PMT cooler power supply.
As the PMT tube cools down, you should see its dark current count rate decrease,
reaching values of less than 10 cps in 30 to 45 minutes. You can monitor the PMT
cool-down with the count rate given by the frequency counter or the computer.
7. Near the end of the PMT cool-down, you can check for light leaks. Close the cover on
the scanning spectrometer if it is open (as shown in Fig. 2), close the entrance slit,
open the internal and exit slits, and finally open the shutter on the exit slit. Turn the
room lights on and off; the PMT signal should remain below 10 cps. If not, you may
have to work with the lights out in the curtained Raman area, and use a flashlight.
When the light check is done, close the shutter on the exit slit.
Alignment of the optical system
1. While the PMT is cooling down, the optical system may be aligned. Remove the two
lenses from their bases. After making sure the shutter on the exit slit is closed, open
the cover to the spectrometer (as shown in Fig. 2) and place a cardboard disk (with
marks indicating its center) over the first mirror (opposite the entrance slit). Open the
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spectrometer entrance slit to almost fully open.
CAUTION: Laser safety goggles must be worn from this point on.
Turn on the He-Ne laser, and adjust its position so that its beam passes through the
center of the entrance slit and falls on the center of the disk at the first mirror. This
defines the optical axis for the spectrometer.
2. Turn on the DMM connected to the Ar ion laser, so it can be used to monitor the laser’s
anode current; use the laser’s manual to determine how the DMM reading relates to
the anode current. Be careful that the current never exceeds 95% of the maximum
rating found in the laser’s manual. You may find that the anode current creeps slowly
with time, so monitor it and adjust as necessary.
Using the instructions in the manual, turn on the Ar ion laser, and adjust the anode
current to its minimum (the Ar laser beam should still be visible). The Ar laser beam
should be deflected upward by the Ar laser mirror, shown in Fig. 2. Adjust the position
of the cardboard pointer (illustrated in Fig. 2) so that both the He-Ne laser beam and
the Ar laser beam are visible near the tip of the pointer. Turn the adjustment screws
on the Ar laser mirror so that the deflected Ar laser beam is vertical and intersects the
He-Ne laser; it may be helpful to adjust the cardboard pointer so that its tip is just
illuminated by both laser beams. A plumb line may be used to see if the deflected Ar
laser beam is vertical. After aligning the Ar laser beam, the cardboard pointer may be
removed.
3. Install the 8.1 cm focal length lens into the lens-holder base nearest the Ar laser beam,
so that the He-Ne laser beam passes through near the center of the lens. Adjust the
base so that the lens is about 10 cm from the Ar laser beam. Use the fine positioning
adjustments on the base so that the He-Ne laser beam again falls on the center of the
first spectrometer mirror.
4. Install the 44.7 cm focal length lens into the second lens-holder base so that the He-Ne
laser beam passes through near the center of this lens. Adjust the base so that this
lens is about 10 cm from the first lens. Use the fine positioning adjustments on the
base so the the He-Ne laser beam again falls on the center of the first spectrometer
mirror. Slide the bases of both mirrors back and forth about 1 cm and see that the
He-Ne laser beam remains near the center of the first spectrometer mirror; if not, the
beam supporting the lens bases must be re-aligned.
5. The Raman sample holder is a metal cylinder (with an inside diameter of about 1 cm,
and an opening on one side) with a stand positioning it at about the same height as the
cardboard pointer tip. Place a strip of cardboard inside the sample holder and position
it so that the deflected Ar laser beam produces a ∼2 cm streak along the cardboard
strip which can be seen when viewed through the opening in the holder; the opening
should be facing the lenses. Adjust the positions of the lens bases so that this streak
is imaged at the spectrometer entrance slit. This should maximize the amount of light
which falls on the first spectrometer mirror. The optical system is now aligned.
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Calibration of the scanning spectrometer
Before doing any Raman scattering experiment, it is necessary to become familiar with the
scanning spectrometer, and to calibrate the scanning motor counter against the wavelength.
The calibration can be accomplished using the He-Ne laser, the Ar laser, and the known
lines from a mercury (Hg) lamp. A calibration graph should be made for future reference.
The slope of the calibration line should be a ratio of two small integers. The reason is that
at some point a grating inside the spectrometer was replaced; the old and new gratings had
standard line densities, but they were two different standards, which differed by the ratio of
two small integers.
1. Read the manual for the scanning spectrometer. The scanning is accomplished by
a precision motor drive, which is controlled by a unit external to the spectrometer.
Always make measurements in the same scan direction to minimize the effects of screw
lag. Note that starting or stopping the motor drive at high speed can ruin your
calibration; always start or stop the drive at low speeds (approximately 10 on the
control unit dial read-out), slowly accelerating or decelerating to the desired speed.
Note that the scanning spectrometer needs electrical power, and it is turned on by
plugging its power cord into a wall socket or switched outlet; when shutting down the
experiment, remember to turn off the scan control unit. When the scan control is on,
lights near the scanning motor counter on the side of the spectrometer should turn on.
If the counter lights are off when the spectrometer is on, then use a DMM to check
connections, bulbs, etc. in the vicinity of the counter.
2. Using a strip of paper, trace the light from the Ar laser (which should be present
as a result of the optics alignment procedure) through the first monochromator; it
may be necessary to turn off the room lights. Run the spectrometer’s scanning motor
(rotating the gratings) so that the scattered light from the Ar laser falls on the internal
slit. Note the scanning motor counter reading; this gives the approximate position for
the subsequent calibration at this wavelength.
Note that the Ar ion laser may be tuned to produce different wavelengths of light. From
the Raman Handbook, find the relative intensities, wavelengths and the coresponding
colors of the four strongest lines for an Ar ion laser.
In order to identify the spectral lines from the various claibration light sources, it
may be useful to use the following table which gives common color names for different
wavelengths of light:
wavelength range (nm) color name
400.0 - 424.0 violet
424.0 - 491.2 blue
491.2 - 575.0 green
575.0 - 585.0 yellow
585.0 - 647.0 orange
647.0 - 700.0 red
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3. Close the cover of the scanning spectrometer, and set the internal and exit slit openings
to 0.3 mm. Close the entrance slit opening to zero. The PMT cool-down data acquisition
on the computer should have completed. [You should save the cool-down data
for your lab report.] Restart the computer data acquisition as for the PMT cool-down,
and recheck that the PMT has cooled down so that its dark current is below 10 cps.
Open the shutter on the exit slit, and slowly open the entrance slit; the count rate from
the PMT should increase. Use the scanning spectrometer motor drive to position the
monochromator for a maximum count rate. Increase the entrance slit opening until
0.3 mm is reached, or the count rate reaches 10 5 cps, whichever comes first.
CAUTION: Never let the PMT count rate exceed 10 6 cps.
4. Rewind the spectrometer scanning motor away from the approximate position for the
Ar laser wavelength so that the count rate drops to the background rate (near or at
the dark current rate). For the calibration you will want to scan through the Ar laser
wavelength to acquire a fully resolved peak. You will want to use a position where
the PMT is at the background rate as your desired starting position for the scanning
motor counter when initiating the calibration scan; make a note of this desired starting
position.
NOTE: The resolution of the spectrometer depends critically on the width of the
various slits in the spectrometer and, in general, reducing the slit width will both
increase the resolution and decrease the signal to the PMT. Also, weak peaks, possibly
next to a strong peak, will require longer scan times. These aspects should be kept in
mind when the scans for Raman lines are performed.
To obtain good calibration peaks, you should try different values for the spectrometer
scanning motor speed and the slit openings.
Concerning the computer data acquisition, it should be noted that the PMT pulse rate
(the “y-axis” of a spectrum) is recorded by the computer, but there is no connection
between scanning counter (the “x-axis” of a spectrum) and the computer. The x-axis
is obtained by assuming that both the wavelength scanning (represented by the counter
advance) and the data acquisition by the computer occur at constant rates in time. In
the computer program you will enter the counter reading when data acquisition begins
(Spectrometer Start Reading) and the counter reading when the data acquisition ends
(Spectrometer End Reading); with this information, the computer program is able to
convert its time sequence of data into a list of PMT pulse rates versus counter reading.
To take data for a spectrum, open the raman datacq jdm.vi program (if it is not
open already). With the spectrometer scanning motor running, enter an approaching
counter reading as the Spectrometer Start Reading; when the counter just reaches the
Start Reading, click the program start arrow. The Spectrometer End reading may be
entered after the program begins running, but it must be entered before clicking the
Stop button. Click on the Stop button when the spectrometer counter just reaches
the End Reading. The program will not automatically stop at the end reading. If you
leave the experiment unattended and find that you have overshot the End Reading,
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then enter an approaching counter reading and click Stop when the new End Reading
is reached. After stopping the scan, the program will prompt for a filename for storing
the data; the file will have two columns of numbers: the counter reading (as calculated
by assuming constant counter and data acquisition rates) and the PMT pulse rate
(proportional to the scattered light intensity).
Repeat the calibration procedure above replacing the light from the Ar ion laser with light
from the He-Ne laser and several strong lines (two yellows and a green) of known wavelength
from a Hg lamp. Set up the Hg lamp so that it shines strongly into the spectrometer entrance
slit. When scanning a new spectrum, you can clear a previous scan in the computer program
display by following the directions below the program display.
CAUTION: Ultraviolet light from the Hg lamp is harmful; do not look at it when on, and
keep it covered to avoid accidents.
The calibration will be given as a straight line fit of the actual wavelength of the incident
light as a function of the counter value at the peaks of the calibration data sets. After an
initial calibration with strong, obvious lines, you may want to look for some weaker lines
from the Hg lamp. For some Hg lines and for the Raman measurement, it may be necessary
to increase the spectrometer slit openings.
Note that there are several types of Hg lamps, with different spectra. Also, if the counting
rate is too high (too much light is entering the spectrometer) then you may get too many
unidentifiable lines. If necessary, close down the entrance slit; recall that the maximum
count rate should be below 10 6 cps. Finally note that some strong lines from the Hg lamp
may place second order diffraction peaks in your spectra. An initial calibration with strong,
obvious lines will alleviate line identification problems.
Raman data acquisition
The liquid samples for Raman scattering measurements are placed in transparent vials which
fit into the sample holder. The side of a vial may be partially covered with aluminum foil
to increase the amount of scattered light sent to the lenses. When placing the vials into the
sample holder, some wedging material may be used so that the vials do not shift during data
acquisition. Recall that the light scattered from the samples will have an intense Rayleigh
line in addition to the much weaker Raman sidebands. Adjust the position of the sample
holder (and the position of the sample vial within the holder as well) in order to obtain the
maximum signal from the Rayleigh line.
For the Raman data acquisition scans, the Ar ion laser anode current should be increased to
95% of its maximum rating. Then perform a slow scan of an appropriate range of wavelengths
to look for the Raman lines. If you have trouble finding the Raman lines, you might want
to look up the known values of the wavelength shifts to make sure you are scanning over the
correct range. If you have carefully adjusted the optics, maximized the Ar ion laser output,
and varied the sample vial position and you cannot see Raman lines or they are very faint
and hard to discern, you may want to try adjusting the alignment of the mirrors internal to
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the laser head, but only as a last resort. Consult the lab instructor before attempting this,
as it is easy to throw the laser off its tuning.
INSTRUCTORS ONLY: The B screw in the back of the ion laser (see the manual link
under EQUIPMENT above), if tweaked, will change the frequency of the laser output light.
If only adjustments of the mirror alignments are desired, the A screws and the B screw in
the front of the laser can be very slightly tweaked while monitoring the current draw to
minimize the current (a very slight adjustment makes a huge impact on current draw). To
avoid accidentally blowing the fuse, this adjustment should be done only at low currents
(e.g. 7 A). Once the current draw has been minimized, it can be set back to 10 - 11 A.
References
[1] J. Loader, Basic Laser Raman Spectroscopy (Heyden & Son, London, 1970).
[2] D. Long, Raman Spectroscopy (McGraw-Hill, New York, 1971).
[3] T. R. Gilson and P. J. Hendra, Laser Raman Spectroscopy (Wiley, London, 1970).
[4] A. Anderson, ed., The Raman Effect (Marcel Dekker, New York, 1971).
[5] R. Chang, Basic Principles of Spectroscopy (McGraw-Hill, New York, 1971).
[6] W. Demtroder, Laser Spectroscopy Basic Concepts and Instrumentation (Springer Verlag,
Berlin, 1988).
[7] S. Walker and H. Straw, Spectroscopy (Chapman and Hill, London, 1962) Vol. 2, p. 176.
[8] B. P. Straughan and S. Walker, Spectroscopy (Chapman and Hill, London, 1962), Vol.
2, 198, pg 225-258.
[9] C.V. Raman and K.S. Krishnan, Indian J. Phys. 2, 387 (1928).
[10] G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, New
York, 1945).
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